The present invention relates to a method for monitoring the solidification of a liquid composition as it passes from a liquid phase to a solid phase.
The change of a liquid composition from a liquid phase to a solid phase occurs in many chemical reactions such as the polymerization of liquid monomers to a solid polymer, curing of thermoset resins such as epoxy resins or polyimides, and cross-linking of single chain polymers. Monitoring the degree of solidification, i.e., completion, of these exemplary types of reactions is advantageous to ensure that the solid product has the desired chemical and physical properties associated with the completion of the reaction, as well as to minimize the energy and time costs associated with maintaining the process conditions past the point of completion.
For example, precision curing conditions are required in the production of cured thermosettable materials and composite materials containing these cured thermosettable materials that are finding increasing use in a variety of advance materials applications. The precision of the curing process is important because the ultimate mechanical properties of these materials depends primarily on completely and uniformly curing these materials. For instance, in a large molded structure of a thermoset resin, the degree of cure may vary from place to place within the structure due to the lack of homogeneity of the starting materials, the exothermic nature of the curing reaction, the thermodiffusivity of the material, and the thermal characteristics of the curing oven or the geometry of the mold.
A common method for characterizing the degree of cure of thermosettable materials involves the measurement of the glass transition temperature (T.sub.g) by one of a number of different techniques. Examples of these techniques include differential scanning calorimetry and thermoexpansion measurements. The equipment associated with these bulk methods are hard to adapt to local in situ measurements, that are necessary to accurately monitor the curing process throughout a large body.
A technique known as dielectric spectroscopy has undergone development resulting in a method for the in situ measurement of the local dielectric properties of curing materials. A frequency range of 0.005 to 10,000 Hz is used to monitor changes in the dielectric loss factor that have been shown to correlate with different stages of the curing process. Another technique recognizes that the intensity and wavelength of fluorescence of certain molecules are dependent upon the viscosity of the environment surrounding the molecules. Because the viscosity of a composition changes with the degree of cure, it is possible to monitor the degree of cure by indirectly monitoring the change in the viscosity of the thermosettable composition and detecting the resultant effect on the fluorescence of the molecules. In still another technique, the attenuation and phase velocity of ultrasonic waves in thermosettable materials is known to vary during the curing process primarily due to the change in viscosity of the materials. An ultrasonic spectral analysis technique has been used to measure the frequency dependence of the ultrasonic velocity and attenuation during the curing process of a thermosettable composition at a fixed temperature.
The prior techniques show the onset of the curing reaction in thermosettable materials. However, during the curing process, properties of the thermosettable material, such as viscosity, conductivity and glass transition temperature, change drastically at the onset of curing and then saturate at an upper bound as the curing process continues and nears completion. For example, as methylmethacrylate polymerizes and cures, the instantaneous glass transition temperature changes from -102.8.degree. to 70.degree. C., and the viscosity rises from about 0.1 to 1.times.10.sup.11 centipoise. Using a fluorescent probe method, a fluorescent probe molecule added to the methylmethacrylate system exhibits an intensity change from 4 to 75 relative units. Unfortunately, by the time the glass transition temperature has reached 40.degree. C., the viscosity has reached only 2.times.10.sup.4 centipoise and the fluorescent intensity has already reached 72.5. The final stages of the curing process must be measured by the remaining 2.5 relative units of the fluorescent intensity and therefore the final stages of the curing process are not observable with adequate resolution. The glass transition temperature and dielectric spectroscopy methods of monitoring the curing of thermosettable resin compositions also suffer from similar limitations, in addition, both these methods are not readily adaptable to in situ measurements of large structures, where the monitoring of the solidification process is most important.